Orthogonal frequency division multiplexing (OFDM) is an example of multi-carrier modulation techniques of converting input serial data into parallel data and transmitting them via a plurality of subcarriers. With the use of OFDM, it is possible to maintain orthogonality between adjacent subcarriers, thereby allowing increased bandwidth efficiency.
However, channel distortion generally occurs in an OFDM signal transmitted via a multi-path channel due to fading. To compensate for the channel distortion due to fading, a receiving side may include an equalizer, and channel estimation that estimates a plurality of subchannels may be provided so that the equalizer can compensate for the channel distortion.
FIG. 1 is a block diagram of a conventional OFDM receiver 10. Referring to FIG. 1, the receiver 10 includes an antenna 15, a tuner 20, an analog-to-digital converting (ADC) block 30, a Fast Fourier Transform (FFT) block 40, an equalization block 50, and a QAM demodulator 60. The tuner 20 tunes to channel frequency of a signal to be received via the antenna 15, and converts the tuned signal into a signal having a base-band frequency. The ADC block 30 converts the analog signal having the base-band frequency into a digital signal. The FFT block 40 performs an FFT operation on the digital signal and outputs the operation result.
The equalization block 50 includes a channel estimation block 51 and an equalizer 52. The channel estimation block 51 receives a signal from the FFT block 40 and estimates channel information, e.g., a channel transfer function. The equalizer 52 compensates for channel distortion due to fading, based on the channel information received from the channel estimation block 51. The QAM demodulator 60 receives a signal from the equalizer 52 and performs QAM demodulation on the signal.
To estimate and compensate for channel distortion due to a multi-path channel, a transmitter transmits a pilot signal (which is also referred to as a “pilot symbol”), and a receiver receives the pilot signal and estimates channel circumstances (or a channel response) by using the received pilot signal. Here, the pilot signal is known to both the transmitter and the receiver. An interpolation method has been introduced to perform channel estimation by using the pilot signal. Examples of the interpolation method are a linear interpolation method and a Gaussian interpolation method.
An OFDM communication system divides an available frequency band into N subchannels, and may use a specific number of subchannels of the N subchannels to transmit the pilot signal. The pilot subchannels may be dispersed among data subchannels.
It is possible to estimate a channel frequency response of the dispersed pilot subcarriers, and to estimate a channel frequency response of data subcarriers according to an interpolation method, e.g., the linear interpolation method. That is, for overall channel estimation, the receiver can extract pilot signals from a plurality of received subchannels, filter the extracted pilot signals with a low-pas filter, and estimate the characteristics of the data subchannels.
Thus, if the low-pass filter is properly designed for channel estimation, the above channel estimation method based on an interpolated signal can be very efficient.
However, when the pilot signals are exposed to noise or the degree of a channel response is small, the performance that estimates the data subchannels by using the low-pass filter may be degraded due to ripple and/or other effects.